[ARM, thumb] Fix disassembling bug after reloading a symbol file

The speed optimization from commit 5f6cac4085c95c5339b9549dc06d4f9184184fa6
made GDB skip reloading all symbols when the same symbol file is reloaded.

As a result, ARM targets only read the mapping symbols the first time we
load a symbol file. When reloaded, the speed optimization above will
cause an early return and gdbarch_record_special_symbol won't be called to
save mapping symbol data, which in turn affects disassembling of thumb
instructions.

First load and correct disassemble output:

Dump of assembler code for function main:
    0x0000821c <+0>:     bx      pc
    0x0000821e <+2>:     nop
    0x00008220 <+4>:     mov     r0, #0
    0x00008224 <+8>:     bx      lr

Second load and incorrect disassemble output:

Dump of assembler code for function main:
    0x0000821c <+0>:     bx      pc
    0x0000821e <+2>:     nop
    0x00008220 <+4>:     movs    r0, r0
    0x00008222 <+6>:     b.n     0x8966
    0x00008224 <+8>:     vrhadd.u16      d14, d14, d31

This happens because the mapping symbol data is stored in an objfile_key-based
container, and that data isn't preserved across the two symbol loading
operations.

The following patch fixes this by storing the mapping symbol data in a
bfd_key-based container, which doesn't change as long as the bfd is the same.

I've also added a new test to verify the correct disassemble output.

gdb/ChangeLog:

2019-11-01  Luis Machado  <luis.machado@linaro.org>

	PR gdb/25124

	* arm-tdep.c (arm_per_objfile): Rename to ...
	(arm_per_bfd): ... this.
	(arm_objfile_data_key): Rename to ...
	(arm_bfd_data_key): ... this.
	(arm_find_mapping_symbol): Adjust access to new bfd_key-based
	data.
	(arm_record_special_symbol): Likewise.

gdb/testsuite/ChangeLog:

2019-11-01  Luis Machado  <luis.machado@linaro.org>

	PR gdb/25124

	* gdb.arch/pr25124.S: New file.
	* gdb.arch/pr25124.exp: New file.

Change-Id: I22c3e6ebe9bfedad66d56fe9656994fa1761c485

2 files changed, 84 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.arch/pr25124.S b/gdb/testsuite/gdb.arch/pr25124.S
new file mode 100644
index 0000000..79f82c7
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/pr25124.S
@@ -0,0 +1,35 @@
+/* Test proper disassembling of ARM thumb instructions when reloading a symbol
+   file.
+
+   Copyright 2012-2019 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+	.syntax unified
+	.thumb
+	.text
+	.p2align 2
+	.global	main
+	.thumb
+	.thumb_func
+	.type main, %function
+main:
+	bx	pc
+	nop
+.code 32
+	mov	r0, #0
+	bx	lr
+	.size	main, .-main
diff --git a/gdb/testsuite/gdb.arch/pr25124.exp b/gdb/testsuite/gdb.arch/pr25124.exp
new file mode 100644
index 0000000..656079c
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/pr25124.exp
@@ -0,0 +1,49 @@
+# Copyright 2019 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test proper disassembling of ARM thumb instructions when reloading a symbol
+# file.
+
+if {![is_aarch32_target]} then {
+    verbose "Skipping ARM tests."
+    return
+}
+
+standard_testfile .S
+
+if { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable debug] != "" } {
+    untested "failed to compile"
+    return -1
+}
+
+gdb_exit
+gdb_start
+gdb_reinitialize_dir $srcdir/$subdir
+
+# Load the symbol file the first time.
+gdb_load ${binfile}
+
+# Check if the disassemble ouput is correct.
+gdb_test "x /i main+8" \
+	"$hex <main\\+8>:\[ \t\]+bx\[ \t\]+lr" \
+	"disassemble thumb instruction (1st try)"
+
+# Reload the symbol file to trigger the bug.
+gdb_load ${binfile}
+
+# Check if the disassemble output is the same as above.
+gdb_test "x /i main+8" \
+	"$hex <main\\+8>:\[ \t\]+bx\[ \t\]+lr" \
+	"disassemble thumb instruction (2nd try)"